One concept for increasing the capacity of optical storage media is to use holographic data storage. In this case the whole volume of the holographic storage medium is used for storing information, not just a few layers as for conventional optical storage media. In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam is modulated by a spatial light modulator and carries the information to be recorded in the form of data pages.
For holographic data storage, but also for other types of optical data storage, photo-sensitive materials such as photopolymers are used for optical data recording. These materials change specific physical properties, e.g. the refractive index, in dependence on the total light energy that is locally absorbed by the material. These changes allow to record data within the material. For some materials it is necessary to pre-expose the material before data can be efficiently recorded, and to expose the material again after recording data. This last exposure or post-exposure is also known as curing, fixing or flood curing. Pre-exposure is necessary to increase the sensitivity of the material, which is needed for achieving a high recording data rate. Curing is necessary to process all unprocessed material after recording, i.e. the recorded data is fixed and the recording of additional data in the material is prevented. It is desirable that pre-exposure and curing do not lead to any detectable data structure in the material. The aim of pre-exposure is to raise the sensitivity of the material before storing data. The aim of curing is to lower the sensitivity of the material after recording data. For both pre-exposure and curing the volume of the photo-sensitive material where data is to be or has been recorded needs to be exposed to light in such a way that the resulting changes of the physical properties do not disturb the data recording quality. Usually this is achieved by exposing the material to incoherent light, which is emitted, for example, by an array of LEDs. In this way a homogeneous change of the physical properties is obtained throughout the recording volume. Using incoherent light sources an inhomogeneous exposure of the material, caused for example by interference effects, is avoided.
For example, U.S. Pat. Nos. 4,799,746 and 4,687,720 disclose methods for curing a photo-sensitive material used for holographic data storage. In both cases a coherent light source like a laser diode is used for optical data recording. An additional source of incoherent light is used for curing. As a consequence two at least partially separate optical paths and two separate electronic drivers to control the light sources are needed. This complicates the optical system and raises its cost.